Ethylene biosynthesis: processing of a substrate analog supports a radical mechanism for the ethylene-forming enzyme.
BACKGROUND
The chemical mechanism of the final step of ethylene biosynthesis (the conversion of 1-aminocyclopropanecarboxylic acid, ACC, to ethylene by ACC oxidase, the ethylene-forming enzyme, EFE) is poorly understood. Two possibilities have been suggested: a radical mechanism and an N-hydroxylation mechanism. We investigated reaction pathways available to radical intermediates in this reaction using an ACC analog, 1-aminocyclobutanecarboxylic acid (ACBC) as a substrate.


RESULTS
ACBC was converted to dehydroproline (delta 1-pyrroline-2-carboxylic acid) by the EFE via a ring expansion process. The possibility that an N-hydroxy-aminoacid (produced during two-electron oxidation) acts as an intermediate in this process was eliminated by control experiments. Chemical model reactions involving two-electron oxidants, such as a positive halogen (X+), which presumably generate N-halo derivatives, produce only decarboxylation products. Radical-based oxidants, in contrast, generate dehydroproline. Model reactions involving sequential single-electron transfer mechanisms also produce dehydroproline; thus our results support the proposal that the EFE-catalyzed step of ethylene biosynthesis proceeds using a radical-based mechanism.


CONCLUSIONS
Our results provide support for a radical mechanism in the final step of ethylene biosynthesis and refute an alternative N-hydroxylation mechanism. This work extends the idea that the intrinsic chemical reactivity of a high energy iron-oxo intermediate can account for the observed products in ethylene biosynthesis.
Pirrung
M C
MC
Department of Chemistry, P.M. Gross Chemical Laboratory, Duke University, Durham, North Carolina 27708-0346, USA.
Cao
J
J
Chen
J
J
D000596:Amino Acids / Q000737:chemistry* / Q000378:metabolism; D000598:Amino Acids, Cyclic; D005030:Ethylenes / Q000096:biosynthesis* / Q000737:chemistry; D008190:Lyases / Q000378:metabolism*; D010944:Plants / Q000201:enzymology; D013379:Substrate Specificity
D000596:Amino Acids; D000598:Amino Acids, Cyclic; D005030:Ethylenes; C016987:1-aminocyclobutanecarboxylic acid; C036216:ethylene; D008190:Lyases; C034693:ethylene forming enzyme